The rapid intracellular degradation of regulatory proteins has been recognized as an important element in the control of vital cellular processes such as the response to heat shock and other stresses, the timing of cell cycle events, the repair of DNA damage, and the adaptation to nutritional and environmental conditions. Research over the last year has focused on the biochemical properties and in vivo functions of a major ATP-dependent protease from E. coli, Clp protease. The clpP gene, coding for the proteolytic subunit of this two-component protease, has been sequenced and shown to be unique. Although not closely related to any known proteases, Clp P has extensive amino acid homology to a protein of unknown function encoded by plant chloroplasts DNA. Immunochemical screening indicates the presence of proteins closely related to Clp P in a variety of other bacteria, yeast and other lower eukaryotes, and in animal cells. Clp P belongs to the serine protease family and two active site residues, Ser111 and His136, have been identified. Mutational alteration of these residues eliminates proteolytic activity while allowing proper folding of Clp P and binding interaction with the regulatory component, Clp A. In vitro studies with purified Clp protease revealed that Clp P has a very weak endoproteolytic activity with low molecular weight substrates and is activated >100 fold by Clp A and ATP. Clp cleaves a number of different peptide bonds and the specificity is not affected by the regulatory components. About two molecules of ATP are hydrolyzed per peptide bond cleaved. Electron micrographs of Clp P show a structure composed of two superimposed hexagonal rings, which bears a striking resemblance to the ATP-dependent multicatalytic protease found in eukaryotic cells.